I found my grandpa’s unfinished painting covered in spiderwebs in the attic.
クモ is a spider. Add a nest to it, and you have a spider nest. What do spider's call their nests? They call it a spiderweb (that is, if they can talk).
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.